(1) Field of the Invention
The present invention has application in the field of the image wise development of images and is suitable where there is an application of pressure as in the case of no carbon required paper, the application of heat as in thermal image transferal systems and the selected image wise application of an electrical discharge.
(2) Description of the Prior Art
Within the field of image transferal it is known to use various dye precursors which are colourless or essentially colourless compounds and which undergo colour reaction when brought into contact with an acidic electronic acceptor material such as attapulgite, bentonite, benzoic acid and such like materials. Use has been made to produce colourless carbon papers and business manifold forms, thermally activated papers and various photographic systems. While many of these dye precursor imaging elements have been commercially successful they have, nevertheless, some disadvantages such as:
1. Precursors which develop the colour rapidly also tend to fade rapidly.
2. Fade resistant dye precursors are very slow at developing colours and are expensive.
3. Such dye precursors are difficult to dissolve and require expensive solvents.
4. The colour range is limited.
Typical of such systems are those of U.S. Pat. No. 4,012,554 which refers to any acid colourable basic chromogenic dye precursors of which the colour blocked triphenylmethane dyes are the best known (using for example potassium cyanide which converts the coloured dyestuff to colourless compounds) and British Pat. No. 1,471,445 which deals with azo or azine dyes which are reactive dyestuffs of a different category.
Solvent dyestuffs of the type with which the present invention is concerned are the free bases of basic or cationic dyes. They are weakly coloured dyes which develop a stronger colour when reacted with a fatty acid. These dyes form a distinct class of oil soluble dyes. While reduction of a triphenylmethane dye will form a leuco base, the free base of basic cationic dyes (i.e. solvent dyes) including triphenylmethanes, xanthenes and etc. are formed by reacting the dyestuff with alkali.